1. Field of the Invention
This invention relates to electrode systems which can be surgically implanted in the human body and attached to nerve bundles to provide stimulation for those nerves as a means for treating nervous system disorders such as seizures or for other purposes relating to diagnostic or therapeutic techniques.
2. Discussion of the Related Art
The use of electrical pulse stimulation of human peripheral nervous system elements has become an important area of research and practical application for both diagnostic and therapeutic techniques. Implanted electrodes have long been used in the field of cardiac pacemaker implants to monitor and control irregular heartbeat and other cardiac related disorders. More recently, implantable electrodes have been used in humans to treat neurological disorders stemming from paralysis, spasms and other dysfunctional states of organs innervated by certain nerve bundles.
Other applications for implantable electrodes relate to their use as recording devices to monitor nervous system activity and in certain instances to provide electrical pulse stimulation in response to such sensed activity.
One further application, as described in U.S. Pat. No. 4,702,254, relates to the periodic stimulation of the vagal nerve to reduce or eliminate seizure disorders such as are associated with epilepsy.
Due to the nature of the electrical activity associated with nerve action, and the need to provide closely coupled, spaced apart electrodes which surround the target nerve bundle, electrodes with a wound helical structure were developed. One such electrode construction is disclosed in U.S. Pat. No. 4,590,946. Electrodes of this design provide a pair of electrode contacts embedded in the inner face of a flexible, helically wound substrate material. In these helical electrodes, the conductors or contacts are connected to an electrical source for monitoring or providing electrical stimulation by one or more pairs of lead wires.
Due to the intricacy of the implant surgery involved, it is necessary that implantable electrodes be relatively easy to attach and conform to the shape of the target nerve bundle. The helical electrode construction has proven to be particularly suitable in that it can be implanted by applying it to the nerve with a corkscrewing action and its spring-like structure naturally conforms to the outer surface of the nerve bundle. These electrodes are necessarily delicate, however, and care must be taken to adequately secure the electrodes to the nerve bundle without effecting the electrode's performance.
Typically, implantable electrodes are helically wound with an inner diameter of approximately 2 mm and an individual helix length of about 3.5 mm, with an overall helix length of 14 mm and total electrode length of 40 cm. The helical substrate is composed of biocompatible silicone rubber and the electrode elements are formed of platinum ribbon which is in some cases only 0.0001 inches thick. The lead wires attach to the electrode elements. The lead wires are typically stainless steel with silicone rubber insulation. The lead wires are attached to the electrode elements with a solder or weld connection.
Implanted electrodes are made of flexible resilient material to accommodate the movement of the nerve bundle itself and the movement of the nerve bundle relative to surrounding tissue. Since the electrode lead wires are attached to the nerve via the electrode structure but also engage the surrounding tissue, any relative movement between the nerve bundle and the surrounding tissue imparts strain on the junction between the lead wires and the helical electrode, and on the nerve itself. Any mechanical forces transmitted to the nerve via the lead wires can cause damage to the nerve or dislocation of the electrode.
To alleviate this problem, prior electrode systems have used a variety of strain-relief elements. U.S. Pat. No. 4,590,946 disclosed a helical electrode with a strain relief tab which was formed around the lead wires and anchored to body tissue such as bone, fascia or other tissue by screws, sutures or other surgical fasteners. While this configuration may have relieved strain imposed by remote forces acting on the lead wires, it did not alleviate the strain on the lead wire to electrode junction which occurred as a result of movement of the nerve relative to the anchoring tissue.
U.S. Pat. No. 4,394,866 disclosed an atrial pacemaker design wherein the lead wires were ligated in place by attachment to an adjacent vein. This design also failed to alleviate strain on the lead wire junction which resulted from relative movement of the anchoring tissue vis-a-vis the electrode implant site.